Conventional rotating fans are efficient for moving large volumes of air. However, disadvantages are associated with the use of conventional rotating fans where small volume flow rates are required, particularly when such fans are used for cooling electronic components.
The advent of solid-state oscillating fans of the type having one or more "flapping" blades provides significant advantages over conventional rotating fans. Solid-state fans of the type described typically employ a piezoelectric crystal which joins the root section of two cantilevered blades. A fan of this type is manufactured by Piezo Electric Products, Inc., Advanced Technology Group, 186 Massachusetts Avenue, Cambridge, Mass. 02139, under the model numbers LP/1200, 2400. Another type of solid-state "flapping blade fan" is described in U.S. Pat. No. 4,498,851, issued to Kolm et al. and assigned to Piezo Electric Products, Inc.
When compared to a conventional rotating fan of similar air volume output, the solid-state fan of the type described above has a number of advantages. At the same flow velocity of a conventional rotating fan, the solid state fan typically consumes approximately one-fifth the current and one-half the power while delivering approximately one-half the flow volume. The solid-state fan also generates substantially less electromagnetic interference than does the electric motor of the conventional fan. Life expectancy of a conventional fan is on the order of 1,000 hours, whereas life expectancy for a piezoelectric fan may be indefinite.
The airflow from an unducted solid-state fan having two opposed, counter-oscillating fan blades is directional but highly turbulent. A single oscillating blade fan also produces directional, turbulent airflow. The fan operates in a quadrature mode of vibration (i.e., vibrating like a diving board with maximum amplitude at the leading edge of each blade) and displaces air through a phenomena known as "vortex shedding." During oscillation of the blades, counterrotating vortices are formed which have substantial angular momentum. The angular momentum is sufficient to prevent an air vortex formed by movement of a fan blade in one direction from counterrotating and being drawn into the vacuum formed behind the blade when the blade direction is reversed. Instead, the first formed vortex is driven away from the reversing blade by the formation of a counterrotating vortex. The vortices are shed in a direction substantially parallel to the rest position of the blade with a small component of velocity which is transverse to the plane of the blade.
Solid-state fans of the type described above have proven to be particularly useful for cooling components on printed circuit boards. However, in some applications, an airflow is desired which is substantially laminar rather than turbulent. In other applications, a ducted airflow is desired. Therefore, a need exists for a method and apparatus for providing substantially laminar airflow from directional, turbulent fluid flow as generated by oscillating or "flapping" fan blades. A need also exists for efficiently ducting airflow from fans of this type.